1. Field of the Invention
The present invention relates to a gear train of an automatic transmission for vehicles which provides at least eight forward speeds by combining four planetary gear sets with three clutches and three brakes.
2. Description of the Related Art
A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A gear train of such an automatic transmission that includes the plurality of planetary gear sets changes rotation speed and torque received from a torque converter of the automatic transmission, and accordingly transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed, and therefore a vehicle can have economical fuel mileage and better performance. For that reason, an automatic transmission that is able to realize more shift speeds is under continuous investigation.
In addition, with the same number of speeds, features of a gear train, such as durability, efficiency in power transmission, and size, substantially depend on the layout of the combined planetary gear sets. Therefore, designs for a combining structure of a gear train are also under continuous investigation.
A manual transmission that has too many speeds causes inconvenience to a driver. Therefore, the advantageous features of having more shift-speeds are more important in an automatic transmission because an automatic transmission automatically controls the shifting operations.
Currently, four-speed and five-speed automatic transmissions are most often found on the market. However, six-speed automatic transmissions have also been realized for enhancement of performance of power transmission and for enhanced fuel mileage of a vehicle. In addition, seven-speed automatic transmissions and eight-speed automatic transmissions have been developed at a good pace.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.